Lithographic projection apparatus and reflector assembly for use therein

ABSTRACT

A lithographic projection apparatus includes a grazing incidence collector. The grazing incidence collector is made up of several reflectors. In order to reduce the amount of heat on the collector, the reflectors are coated. The reflector at the exterior of the collector has an infrared radiating layer on the outside. The inner reflectors are coated with an EUV reflective layer on the outside.

RELATED APPLICATION

This application claims the benefit of priority to European PatentApplication No. 02078528.3, filed Aug. 27, 2002, the contents of whichare herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic projection apparatus anda reflector assembly for use therein.

2. Description of the Related Art

The term “patterning device” as here employed should be broadlyinterpreted as referring to device that can be used to endow an incomingradiation beam with a patterned cross-section, corresponding to apattern that is to be created in a target portion of the substrate. Theterm “light valve” can also be used in this context. Generally, thepattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). An example of such a patterning device is amask. The concept of a mask is well known in lithography, and itincludes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

Another example of a patterning device is a programmable mirror array.One example of such an array is a matrix-addressable surface having aviscoelastic control layer and a reflective surface. The basic principlebehind such an apparatus is that, for example, addressed areas of thereflective surface reflect incident light as diffracted light, whereasunaddressed areas reflect incident light as undiffracted light. Using anappropriate filter, the undiffracted light can be filtered out of thereflected beam, leaving only the diffracted light behind. In thismanner, the beam becomes patterned according to the addressing patternof the matrix-addressable surface. An alternative embodiment of aprogrammable mirror array employs a matrix arrangement of tiny mirrors,each of which can be individually tilted about an axis by applying asuitable localized electric field, or by employing piezoelectricactuators. Once again, the mirrors are matrix-addressable, such thataddressed mirrors will reflect an incoming radiation beam in a differentdirection to unaddressed mirrors. In this manner, the reflected beam ispatterned according to the addressing pattern of the matrix-addressablemirrors. The required matrix addressing can be performed using suitableelectronics. In both of the situations described hereabove, thepatterning device can comprise one or more programmable mirror arrays.More information on mirror arrays as here referred to can be seen, forexample, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and WO 98/38597and WO 98/33096. In the case of a programmable mirror array, the supportstructure may be embodied as a frame or table, for example, which may befixed or movable as required.

Another example of a patterning device is a programmable LCD array. Anexample of such a construction is given in U.S. Pat. No. 5,229,872. Asabove, the support structure in this case may be embodied as a frame ortable, for example, which may be fixed or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table. However, the general principles discussed in such instancesshould be seen in the broader context of the patterning device ashereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningdevice may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(e.g. comprising one or more dies) on a substrate (silicon wafer) thathas been coated with a layer of radiation-sensitive material (resist).In general, a single wafer will contain a whole network of adjacenttarget portions that are successively irradiated via the projectionsystem, one at a time. In current apparatus, employing patterning by amask on a mask table, a distinction can be made between two differenttypes of machine. In one type of lithographic projection apparatus, eachtarget portion is irradiated by exposing the entire mask pattern ontothe target portion at once. Such an apparatus is commonly referred to asa wafer stepper. In an alternative apparatus, commonly referred to as astep-and-scan apparatus, each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection. Since, in general, the projection system will have amagnification factor M (generally<1), the speed V at which the substratetable is scanned will be a factor M times that at which the mask tableis scanned. More information with regard to lithographic devices as heredescribed can be seen, for example, from U.S. Pat. No. 6,046,792.

In a known manufacturing process using a lithographic projectionapparatus, a pattern (e.g. in a mask) is imaged onto a substrate that isat least partially covered by a layer of radiation-sensitive material(resist). Prior to this imaging, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. It is important to ensure that the overlay (juxtaposition) of thevarious stacked layers is as accurate as possible. For this purpose, asmall reference mark is provided at one or more positions on the wafer,thus defining the origin of a coordinate system on the wafer. Usingoptical and electronic devices in combination with the substrate holderpositioning device (referred to hereinafter as “alignment system”), thismark can then be relocated each time a new layer has to be juxtaposed onan existing layer, and can be used as an alignment reference.Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens.” However, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and6,262,796.

In a lithographic apparatus the size of features that can be imaginedonto the substrate is limited by the wavelength of the projectionradiation. To produce integrated circuits with a higher density ofdevices, and hence higher operating speeds, it is desirable to be ableto image smaller features. While most current lithographic projectionapparatus employ ultraviolet light generated by mercury lamps or excimerlasers, it has been proposed to use shorter wavelength radiation in therange 5 to 20 nm, especially around 13 nm. Such radiation is termedextreme ultraviolet (EUV) or soft x-ray and possible sources include,for instance, laser-produced plasma sources, discharge plasma sources,or synchrotron radiation from electron storage rings. Apparatus usingdischarge plasma sources are described in: W. Partlo, I. Fomenkov, R.Oliver, D. Birx, “Development of an EUV (13.5 nm) Light Source Employinga Dense Plasma Focus in Lithium Vapor”, Proc. SPIE 3997, pp. 136–156(2000); M. W. McGeoch, “Power Scaling of a Z-pinch Extreme UltravioletSource”, Proc. SPIE 3997, pp. 861–866 (2000); W. T. Silfvast, M.Klosner, G. Shimkaveg, H. Bender, G. Kubiak, N. Fornaciari, “High-PowerPlasma Discharge Source at 13.5 and 11.4 nm for EUV lithography”, Proc.SPIE 3676, pp. 272–275 (1999); and K. Bergmann et al., “HighlyRepetitive, Extreme Ultraviolet Radiation Source Based on aGas-Discharge Plasma”, Applied Optics, Vol. 38, pp. 5413–5417 (1999).

EUV radiation sources may require the use of a rather high partialpressure of a gas or vapor to emit EUV radiation, such as dischargeplasma radiation sources referred to above. In a discharge plasmasource, for instance, a discharge is created in between electrodes, anda resulting partially ionized plasma may subsequently be caused tocollapse to yield a very hot plasma that emits radiation in the EUVrange. The very hot plasma is quite often created in Xe, since a Xeplasma radiates in the Extreme UV (EUV) range around 13.5 nm. For anefficient EUV production, a typical pressure of 0.1 mbar is requirednear the electrodes to the radiation source. A drawback of having such arather high Xe pressure is that Xe gas absorbs EUV radiation. Forexample, 0.1 mbar Xe transmits over 1 m only 0.3% EUV radiation having awavelength of 13.5 nm. It is therefore required to confine the ratherhigh Xe pressure to a limited region around the source. To reach thisthe source can be contained in its own vacuum chamber that is separatedby a chamber wall from a subsequent vacuum chamber in which thecollector mirror and illumination optics may be obtained.

Thermal radiation emanating from, among others, the EUV source and thefoil trap in a lithographic projection apparatus results in heating ofthe objects on which it impinges. In a lithographic projection apparatusthese objects will generally be the optical components which make up theapparatus. An example of an optical component placed in the vicinity ofthe source, may be formed by a set of reflectors which function as acollector for light emanating from the source. Heating up of thecollector due to this thermal radiation leads to expansion of parts inthe collector causing geometrical aberrations of the collector and,ultimately, leads to its destruction.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide a lithographicprojection apparatus with a reflective element, in particular acollector, for which the radiative heat load is reduced.

It is another aspect of the present invention to provide a lithographicprojection apparatus with a reflective element, in particular acollector, that is effectively cooled.

It is yet another aspect of the present invention to provide a collectorwith a relatively long life span.

This and other aspects are achieved according to the present inventionin a lithographic apparatus an illumination system configured to form abeam of radiation from radiation provided by a radiation source, asupport configured to hold a patterning device which is to be irradiatedby the projection beam to pattern the beam, a substrate table configuredto hold a substrate, and a projection system configured to image anirradiated portion of the patterning device onto a target portion of thesubstrate, wherein a reflector assembly is placed in the vicinity of thesource or an image of the source, the reflector assembly including atleast an inner and an outer reflector extending in the direction of anoptical axis on which the source or an image of the source is located,the inner reflector being closer than the outer reflector to the opticalaxis, the reflectors each having an inner reflective surface and anouter backing layer, the backing layer of the inner reflector beingcovered with a reflective layer having a reflectivity of between 0.7 and0.99, preferably between 0.8 and 0.99, for wavelengths between 0.1 and100 μm, preferably between 1 and 10 μm. Thus, the reflector assemblywill reflect a substantial amount of the infrared radiation thatimpinges upon the back of the reflector, which will reduce the heat loadon the reflector assembly.

In another embodiment of the invention, a reflector assembly is placedin the vicinity of the source or an image of the source, the reflectorassembly comprising at least an inner and an outer reflector extendingin the direction of an optical axis on which the source or an image ofthe source is located, the inner reflector being closer to the opticalaxis than the outer reflector, the reflectors each having an innerreflective surface and an outer backing layer, the backing layer of theouter reflector being covered with a radiative layer having anemissivity of, typically 0.8, for wavelengths between 0.1 and 100 μm,preferably between 1 and 10 μm. By providing the backing layer of theouter reflector with relatively strong infrared radiation emittingproperties, increased amounts of radiation are emitted by the reflectorassembly resulting in improved radiation cooling.

In yet another embodiment of the invention a reflector assembly isplaced in the vicinity of the source or an image of the source, thereflector assembly comprising at least an inner and an outer reflectorextending in the direction of an optical axis on which the source or animage of the source is located, the inner reflector being closer to theoptical axis than the outer reflector, the reflectors each having aninner reflective surface and an outer backing layer, the backing layerof the outer reflector being covered with a radiative layer having anemissivity of, typically 0.8, for wavelengths between 0.1 and 100 μm,preferably between 1 and 10 μm and the backing layer of the innerreflector is covered with a reflective layer having a reflectivity of,typically 0.9 or more, for wavelengths between 0.1 and 100 μm,preferably between 1 and 10 μm. The reflector assembly thus has both areflective coating on the backing layer of the inner reflectors and aradiative coating on the backing of the outermost reflector, for bothreducing the absorbed heat radiation and increasing the emitted heatradiation.

The reflective layer may be made of a noble metal, such as, for example,gold or ruthenium. The radiative layer may be made of carbon for heatload reductive properties.

Each reflector may comprise at least two adjacent reflecting surfaces,the reflecting surfaces further from the source being placed at smallerangles to the optical axis than the reflecting surface that is closer tothe source. In this way, a grazing incidence collector is constructedfor generating a beam of UV radiation propagating along the opticalaxis. Preferably, at least two reflectors are placed substantiallycoaxially and extend substantially rotationally symmetric around theoptical axis. A grazing incidence collector of this (Wolter-) type is,for instance, described in German patent application, DE 101 38 284.7,which is equivalent to U.S. Patent Application Publication 2003/0095623A1, which is incorporated herein by reference. The collector whichresults can be used as an (E)UV radiation focusing device in alithographic projection apparatus.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. One of ordinary skill in the art willappreciate that, in the context of such alternative applications, anyuse of the terms “reticle”, “wafer” or “die” in this text should beconsidered as being replaced by the more general terms “mask”,“substrate” and “target portion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultraviolet(UV) radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm)and extreme ultra-violet (EUV) radiation (e.g. having a wavelength inthe range 5–20 nm), as well as particle beams, such as ion beams orelectron beams.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings inwhich corresponding reference symbols indicate corresponding parts, andin which:

FIG. 1 schematically depicts a lithographic projection apparatusaccording to an embodiment of the present invention;

FIG. 2. shows a side view of an EUV illuminating system and projectionoptics of a lithographic projection apparatus according to the presentinvention;

FIG. 3 shows a detail of the radiation source and grazing incidencecollector of the present invention;

FIG. 4 shows an axial cross-sectional view of a collector layoutaccording to the present invention;

FIG. 5 shows a perspective side view of an embodiment of the collectorequipped with radiation fins;

FIG. 6 shows a schematic view of an embodiment of a lithographicprojection apparatus having two differential pressure chambers and

FIG. 7 is a detailed view of the structure of a foil trap.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic projection apparatus 1according to an embodiment of the invention. The apparatus 1 includes abase plate BP. The apparatus may also include a radiation source LA(e.g. UV or EUV radiation, such as, for example, generated by an excimerlaser operating at a wavelength of 248 nm, 193 nm or 157 nm, or by alaser-fired plasma source operating at 13.6 nm). A first object (mask)table MT is provided with a mask holder configured to hold a mask MA(e.g. a reticle), and is connected to a first positioning device PM thataccurately positions the mask with respect to a projection system orlens PL. A second object (substrate) table WT is provided with asubstrate holder configured to hold a substrate W (e.g. a resist-coatedsilicon wafer), and is connected to a second positioning device PW thataccurately positions the substrate with respect to the projection systemPL. The projection system or lens PL (e.g. a mirror group) is configuredto image an irradiated portion of the mask MA onto a target portion C(e.g. comprising one or more dies) of the substrate W.

As here depicted, the apparatus is of a reflective type (i.e. has areflective mask). However, in general, it may also be of a transmissivetype, for example with a transmissive mask. Alternatively, the apparatusmay employ another kind of patterning device, such as a programmablemirror array of a type as referred to above.

The source LA (e.g. a discharge or laser-produced plasma source)produces radiation. This radiation is fed into an illumination system(illuminator) IL, either directly or after having traversed aconditioning device, such as a beam expander, for example. Theilluminator IL may comprise an adjusting device configured to set theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in the projectionbeam PB. In addition, it will generally comprise various othercomponents, such as an integrator and a condenser. In this way, theprojection beam PB impinging on the mask MA has a desired uniformity andintensity distribution in its cross-section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus, as is oftenthe case when the source LA is a mercury lamp, for example, but that itmay also be remote from the lithographic projection apparatus, theradiation which it produces being led into the apparatus (e.g. with theaid of suitable directing mirrors). This latter scenario is often thecase when the source LA is an excimer laser. The present inventionencompasses both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through thelens PL, which focuses the beam PB onto a target portion C of thesubstrate W. With the aid of the second positioning device PW andinterferometer(s) IF, the substrate table WT can be moved accurately,e.g. so as to position different target portions C in the path of thebeam PB. Similarly, the first positioning device PM can be used toaccurately position the mask MA with respect to the path of the beam PB,e.g. after mechanical retrieval of the mask MA from a mask library, orduring a scan. In general, movement of the object tables MT, WT will berealized with the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which are not explicitlydepicted in FIG. 1. However, in the case of a wafer stepper (as opposedto a step and scan apparatus) the mask table MT may just be connected toa short stroke actuator, or may be fixed. The mask MA and the substrateW may be aligned using mask alignment marks M₁, M₂ and substratealignment marks P₁, P₂.

-   1. The depicted apparatus can be used in two different modes:    In step mode, the mask table MT is kept essentially stationary, and    an entire mask image is projected at once, i.e. a single “flash,”    onto a target portion C. The substrate table WT is then shifted in    the X and/or Y directions so that a different target portion C can    be irradiated by the beam PB;-   2. In scan mode, essentially the same scenario applies, except that    a given target portion C is not exposed in a single “flash.”    Instead, the mask table MT is movable in a given direction (the    so-called “scan direction”, e.g., the Y direction) with a speed v,    so that the projection beam PB is caused to scan over a mask image.    Concurrently, the substrate table WT is simultaneously moved in the    same or opposite direction at a speed V=Mv, in which M is the    magnification of the lens PL (typically, M=¼ or ⅕). In this manner,    a relatively large target portion C can be exposed, without having    to compromise on resolution.

FIG. 2 shows the projection apparatus 1 comprising an illuminationsystem IL, a source-collector module or radiation unit 3, illuminationoptics unit 4, and projection optics system PL. A radiation system 2comprises the source-collector module or radiation unit 3 and theillumination optics unit 4. The radiation unit 3 is provided with aradiation source LA, which may be formed by a discharge plasma.Referring to FIG. 3, an EUV radiation source 6 may employ a gas orvapor, such as Xe gas or Li vapor in which a very hot plasma may becreated to emit radiation in the EUV range of the electromagneticspectrum. The very hot plasma is created by causing a partially ionizedplasma of an electrical discharge to collapse onto the optical axis O.Partial pressures of 0.1 mbar of Xe, Li vapor or any other suitable gasor vapor may be required for efficient generation of the radiation. Theradiation emitted by radiation source LA is passed from the sourcechamber 7 into collector chamber 8 via a gas barrier or “foil trap” 9.The gas barrier 9 comprises a channel structure such as, for instance,described in detail in U.S. Patent Application Publication 2002/0154279A1 and U.S. Pat. No. 6,359,969.

The collector chamber 8 comprises a radiation collector 10 whichaccording to the present invention is formed by a grazing incidencecollector. Radiation passed by collector 10 is reflected off a gratingspectral filter 11 to be focused in a virtual source point 12 at anaperture in the collector chamber 8. From chamber 8, the projection beam16 is reflected in illumination optics unit 4 via normal incidencereflectors 13, 14 onto a reticle or mask positioned on reticle or masktable MT. A patterned beam 17 is formed which is imaged in projectionoptics system PL via reflective elements 18, 19 onto wafer stage orsubstrate table WT. More elements than shown may generally be present inillumination optics unit 4 and projection system PL.

As can be seen in FIG. 3, the grazing incidence collector 10 comprises anumber of nested reflector elements 21, 22, 23. A grazing incidencecollector of this type is, for example, shown in U.S. Patent ApplicationPublication 2003/0095623 A1.

As indicated in FIG. 4, the infrared radiation 40 impinges on acollector 50 which is aligned along an optical axis 47. The collector 50may comprise several reflectors 42, 43, 46. An example of such acollector is shown in FIG. 3 with reference numeral 10. In FIG. 4, theinner reflector is indicated by reference numeral 42, the outerreflector is indicated by reference numeral 46. In between thereflectors 42 and 46 several other reflectors 43 may be located, theoutlines of which are shown in FIG. 4 with dashed lines. All thereflectors 42 and 43 are coated on their backing layer 52 with aheat/infrared radiation reflecting layer 56, such that infraredradiation 40 on these reflectors is reflected as indicated by the arrows44. The outer reflector 46 has on its backing layer 52 a radiativecoating 62. The arrows 48 in FIG. 4 indicate heat/infrared radiation.

In FIG. 4, further detailed composition of the inner reflector 42 of thecollector 50 is illustrated. The reflector 42 includes a backing layer52 made of material that gives the reflector 42 its mechanical strengthe.g. nickel (Ni) of thickness 0.5 to 1 mm. The reflectors 42 ,43 and 46include an (E)UV reflecting side, in FIG. 4, as an example, shownincluding two parts 58 and 59. On the (E)UV mirroring side 58 of thereflector 42, a coating 54 is added of a material that will give thereflector its requested (E)UV reflecting properties, such as, forexample, gold (Au) or ruthenium (Ru), of thicknesses in the range ofapproximately 50 nanometers to several microns. According to the presentinvention, the manufacturing process of depositing a noble metal layer54 as an (E)UV reflective layer is extended in that on the side 60 ofthe backing layer 52 a further coating 56, such as, for example, gold ofthickness such that in can be considered as infinitely thick for theinfrared radiation, i.e. approximately several microns, or anotherinfrared radiation reflecting material, is added, by known techniquessuch as, for example, chemical vapor or electrochemical deposition.Coating 56 is substantially reflecting for heat/infrared radiation,which results in less heat/infrared absorption of the backing layer 52.

In FIG. 4, the detailed composition of the outer reflector 46 isillustrated. Instead of a heat/infrared mirroring layer 56 that coversthe backing layer 52 as is the case for the inner layers 42, 43, thebacking layer 52 of the outer reflector 46 is covered on the outside 60with a heat/infrared radiative layer 62 made of, for example, carbon(C), several microns thick or any other heat/infrared radiative materialknown to the those of ordinary skill. The carbon coating will enhancethe “black body” emissivity of the outermost reflector 46 and hence ofthe entire collector 50.

The mirroring side 58 of the reflectors 42, 43 and 46 in FIG. 4 may becurved. It may include two joining segments one of which is shaped asthe segment of a hyperbola and one of which is shaped as a ellipsoid.

In FIG. 5, a collector 50 is shown which has on its outer reflector 46several radiation fins 72–75 attached. These radiation fins 72–75 may bearbitrarily distributed on the outer reflector 46. The radiation fins72–75 may increase the heat/infrared “black body” reflecting propertiesof the collector 50 even further.

In another embodiment, an improved vacuum separation between the EUVsource and the optical components further along the optical axis may beachieved by using a collector that is part of a vacuum separation. Thisis realized by pumping the space that separates the collector from theother components in the lithographic projection apparatus. By using areflector as described in U.S. Patent Application Publication2003/0095623 A1, use is made of the relatively high flow resistance ofthe “onion-shell” type collector. The outside of the collector may forma vacuum barrier, while a pump may be employed immediately downstream ofthe reflector for pumping off residual gas passing through the collectorat relatively low pumping rates such as 1 mbar·1/s.

This embodiment will be described with reference to FIG. 6. In FIG. 6,part of an EUV illuminator 71 is shown. A channel array or foil trap 61is provided between EUV source 72 and a collector 63. Due to the limitedflow conductance of the channel array or foil trap 61, the pressurebehind this array can be at least a 100 times lower than at the side ofthe EUV source 72, when a pump speed of several 1000 1/s can be reachedbehind the channel array 61. In view of the close distance of collector63, this pump speed cannot be achieved by pump 67. A channel array 61suitable for use in the present invention has been described in U.S.Patent Application Publication 2002/0154279 A1 and U.S. Pat. No.6,359,696. The collector 63 may be a multi-shell grazing incidence EUVcollector 63 of the type as described in U.S. Patent ApplicationPublication 2003/0095623A1. These two components are connected viacircumferential walls 66, 68 to housing 70 to constitute a vacuumchamber 65. The vacuum chamber 65 is evacuated by a pump 67. Due to thesmall separation 93 between the foil trap 61 and the grazing incidenceEUV collector 63 of a few centimeters, which is kept as small aspossible to limit the size of the EUV illuminator, the pump 67 will notbe able to create a sufficient vacuum in the chamber 65 as the effectivepump speed of pump 67 may be only a few 100 1/s. Therefore, a secondpump 69 is arranged behind the grazing incidence EUV collector 63. Thegrazing incidence EUV collector 63 has a limited flow conductance suchas 200 1/s. The pumps 67 and 69 together create the desired vacuum inthe vacuum chamber 65, at a pump speed of several 100 1/s for pump 67and several 1000 1/s for pump 69.

In FIG. 7 the detailed structure 81 of a part of the foil trap 61 isshown. The structure 81 consists of narrowly spaced slits or narrowelongated channels 83 which together form an open laminar structure.Also, the grazing incidence EUV collector 63 includes, due to its onionlike shell structure, open laminar channels.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1. A lithographic projection apparatus, comprising: an illuminationsystem configured to form a beam of radiation from radiation provided bya radiation source; a support configured to hold a patterning devicewhich is to be irradiated by the beam of radiation to pattern theprojection beam; a substrate table configured to hold a substrate; aprojection system configured to image an irradiated portion of thepatterning device onto a target portion of the substrate; and areflector assembly placed in the vicinity of the source or an image ofthe source, the reflector assembly comprising an inner and an outerreflector extending in a direction of an optical axis on which thesource or an image of the source is located, the inner reflector beingcloser to the optical axis than the outer reflector, the inner and outerreflectors each having an inner reflective surface and an outer backinglayer, the inner reflective surface of the outer reflector facing thebacking layer of the inner reflector, the backing layer of the innerreflector being covered with a reflective layer having a reflectivity ofbetween 0.7 and 0.99 for wavelengths between 0.1 and 100 μm.
 2. Alithographic projection apparatus according to claim 1, wherein thereflective layer comprises a noble metal.
 3. A lithographic projectionapparatus according to claim 2, wherein the noble metal comprises goldor ruthenium.
 4. A lithographic projection apparatus, comprising: anillumination system configured to form a beam of radiation fromradiation provided by a radiation source; a support configured to hold apatterning device which is to be irradiated by the beam of radiation topattern the projection beam; a substrate table configured to hold asubstrate; a projection system configured to image an irradiated portionof the patterning device onto a target portion of the substrate; and areflector assembly placed in the vicinity of the source or an image ofthe source, the reflector assembly comprising an inner and an outerreflector extending in a direction of an optical axis on which thesource or an image of the source is located, the inner reflector beingcloser to the optical axis than the outer reflector, the inner and outerreflectors each having an inner reflective surface and an outer backinglayer, the inner reflective surface of the outer reflector facing thebacking layer of the inner reflector, the backing layer of the outerreflector being covered with a radiative layer having an emissivity ofbetween 0.6 and 0.95 for wavelengths between 0.1 and 100 μm.
 5. Alithographic projection apparatus according to claim 4, wherein theradiative layer comprises carbon.
 6. A lithographic projection apparatusaccording to claim 4, wherein the backing layer of the inner reflectoris covered with a reflective layer having a reflectivity of between 0.7and 0.99 for wavelengths between 0.1 and 100 μm.
 7. A lithographicprojection apparatus according to claim 1 or 4, wherein the inner andouter reflectors are substantially coaxial and extend substantiallyrotationally symmetric around the optical axis.
 8. A lithographicprojection apparatus according to claim 1 or 4, wherein at least theouter reflector comprises radiation fins.
 9. A reflector assemblycomprising: an inner and an outer reflector, the inner and outerreflectors each having an inner reflective surface and an outer backinglayer, the inner reflective surface of the outer reflector facing thebacking layer of the inner reflector, the backing layer of the innerreflector being covered with a reflective layer having an reflectivityof between 0.7 and 0.99 for wavelengths between 0.1 and 100 μm.
 10. Areflector assembly according to claim 9, wherein the reflective layercomprises a noble metal.
 11. A reflector assembly according to claim 10,wherein the noble metal comprises gold or ruthenium.
 12. A reflectorassembly, comprising: an inner and an outer reflector, the inner andouter reflectors each having an inner reflective surface and an outerbacking layer, the inner reflective surface of the outer reflectorfacing the backing layer of the inner reflector, the backing layer ofthe outer reflector being covered with a radiative layer having anemissivity of between 0.7 and 0.99 for wavelengths between 0.1 and 100μm.
 13. A reflector assembly according to claim 12, wherein the backinglayer of the inner reflector is covered with a reflective layer having areflectivity of between 0.7 and 0.99 for wavelengths between 0.1 and 100μm.
 14. A reflector assembly according to claim 12, wherein theradiative layer comprises carbon.
 15. A reflector assembly according toclaims 9 or 12, wherein the outer reflector comprises radiation fins.16. A reflector assembly according to claim 9 or 12, wherein the innerand outer reflectors are substantially coaxial.